Dielectric materials for use in multilayer dielectric systems are subject to severe thermal stresses because of the multiplicity of firing and coating cycles which they undergo during the building up of multilayer structures. In particular they must have good electrical properties such as low sheet capacitance (K&lt;10), high insulation resistance (&gt;1.times.10.sup.12 VDC) and high breakdown voltage (&gt;500 V/mil). Simultaneously, they must throughout the many processing cycles remain thermally conductive, be compatible with the conductor materials used therewith and, most importantly, the adherent layers of substrate and dielectric materials must remain flat upon completion of the firing cycles.
Warpage of ceramic substrates having adherent layers of dielectrics is commonly known as "bowing". One cause of bowing is mismatching of the thermal coefficients of expansion (TCE) of the adherent layers and the substrate. When the TCE of the substrate is substantially less than that of the dielectric layer, bowing in the direction of the dielectric layer away from the substrate takes place. When this occurs, the dielectric layer undergoes severe compressive stresses. On the other hand, when the TCE of the dielectric is substantially less than that of the substrate layer, bowing in the direction of the substrate away from the dielectric layer takes place. When this occurs the dielectric layer undergoes severe tensile stresses.
Though TCE mismatch can be a frequent cause of substrate bowing, the phenomenon is also facilitated by the number and thickness of the dielectric layers and is a function too of the thickness of the substrate.
To the extent substrate bowing is caused mainly by TCE mismatch the problem can often be overcome by the addition to the dielectric of materials the TCE of which more closely match that of the substrate. Such additions must, however, be minimized in order to avoid detrimental effects on the electrical properties of the dielectric material. Commercially available dielectric materials contain as much as 50% wt. of added ceramic oxides such as Al.sub.2 O.sub.3 and SiO.sub.2. But such high concentrations of ceramic oxides reduce the ability of the compositions to sinter and to form a nonporous film when they are fired under Thick Film Processing Conditions and thus tend to degrade the reliability of the dielectric composite materials. In particular leakage current tends to be raised and breakdown voltage tends to be lowered by such additions. Moreover not all bowing problems are due to TCE mismatch in which case the addition of TCE mismatch materials may be of little value. Thus dielectric layers have TCE values which closely match those of the substrate can still incur substrate bowing.
In copending U.S. application Ser. Nos. 885,827 and 885,828 filed concurrently herewith, applicant has proposed the use as dielectric materials of certain amorphous crystallizing glasses which upon firing form a single ceramic crystalline phase. Because of the excellent reliability of these glass ceramic dielectrics it is highly desirable to be able to formulate them so that substrate bowing can be substantially reduced without significantly degrading their excellent electrical properties. More particularly it is essential that the electrical reliability of multilayer system using these unique materials be maintained while at the same time substrate bowing is minimized.